The influence of suspended bottom material (0-25 mg multiplied by 1-1)) in addition to algal cells (Phaeodactylum tricornutum) (0-500 mu g dry org wt multiplied by 1-1)) on clearance, growth and energetics in Spisula subtruncata (da Costa) has been studied. Clearance and respiration rate were independent of concentrations of algae and suspended bottom material. Algal ingestion rate was proportional to algal concentration, and independent of the concentration of suspended bottom material. Growth rate increased with algal concentration, and was further increased by 10 to 110% by the addition of suspended bottom material. The positive effect of suspended bottom material on growth is due to a higher efficiency of assimilation of the ingested algae and/or to the utilization of organic matter in the suspended bottom material. The efficiency of algal assimilation decreased from 76% at a low (150 mu g dry org wt multiplied by 1-1) to 33% at a high (500 mu g) algal concentration in specimens fed pure suspensions of algae. The water processing potential (liters of water cleared/mlO2) consumed) was about 10 in S. subtruncata), in comparison to about 15 in Mytilus edulis. Thus, S. subtruncata requires a luxuriant habitat in comparison to M. edulis. This is consistent with the distribution of S. subtruncata in the Oresund.